1. Field of the Invention
The present invention relates to a system for generating a color proof image or color proof sheet to be checked against the original image, before a printed color document including a half-tone dot image is produced by a color printing machine such as a rotary press or the like.
2. Description of the Related Art
It has heretofore been customary to produce a color proof for examining and correcting colors before a printed color document of a halftone dot image is produced by a color printing press.
Color printers are used to produce color proofs because the color printers are relatively simple in structure and inexpensive to manufacture and can produce hard copies with images formed on sheets a plurality of times in a short period of time since, as well known in the art, they do not need the production of process-plate films and presensitized plates which are required by color printing machines.
FIG. 43 of the accompanying drawings shows the sequence of a conventional process of producing a color proof.
According to the conventional process of producing a color proof, as shown in FIG. 43, an image on an image document 2 is read by an image reader such as a color scanner having a CCD area sensor or the like, and gradation image data Ia of each of the colors R (red), G (green), and B (blue) are generated from the read image in a step F1.
Then, the RGB gradation image data Ia are converted by a color conversion process into halftone dot area percentage data aj of the four plates of respective colors C (cyan), M (magenta), Y (yellow), and K (black), where j=C, M, Y, K) in a step F2. The color conversion process has various versions corresponding to different color printing machines, and those versions are based on know-hows of various printing companies depending on their color printing machines.
Images on printed color documents produced by the color printing machines are halftone dot images. To produce a printed color document actually, halftone dot area percentage data aj produced by a color conversion process are developed into bit map data, and a process-plate film or the like is generated based on the bit map data. Because an automatic image developing machine is required, a process following the generation of the process-plate film is considerably complex.
In order to facilitate the production of a color proof, a digital color printer DP is employed as an image output apparatus. The digital color printer DP forms an image on a donor film by digitally controlling the intensity and time of three-primary emission of LED (light-emitting diode) light or LD (laser diode) light per pixel, and transfers the image from the donor film to an image-receiving sheet to form the image thereon. The digital color printer DP is much more inexpensive than a color printing machine which generates presensitized plates from printing plates and produces a printed color document with the presensitized plates. The digital color printer DP is also smaller in volume and lighter in weight.
In order to employ the digital color printer DP, it is necessary to convert the halftone dot area percentage data aj of the four plates C, M, Y, K produced in the step F2 into image data (called "common color space data") independent of devices including a printing device, a CRT, a photographic device, an LED, etc., e.g., tristimulus value data X, Y, Z.
Therefore, the halftone dot area percentage data aj of the four plates C, M, Y, K are converted into tristimulus value data X, Y, Z by a image data processing in a step F4. The image data processing has heretofore been carried out by a process which uses the Neugebauer's equation.
For such conversion, calorimetric data Xi, Yi, Zi (i represents 2.sup.4 =16 colors for the four plates C, M, Y, K) for the colors of printing inks are measured by a calorimeter. For measuring the calorimetric data Xi, Yi, Zi, the 16 colors are printed on a print sheet which will be used to produce a printed color document by a color printing machine. The 16 colors correspond to the presence and absence of the respective colors C, M, Y, K, which represent a combination of 2.sup.4 =16 colors.
Specifically, the 16 colors include the color W (white) which is present when nothing is printed on the print sheet, the primary colors C, M, Y, the color K (black), and the mixed colors C+M, C+Y, C+K, M+Y, M+K, Y+K, C+M+Y, C+M+K, C+Y+K, M+Y+K, and C+M+Y+K. The colors of reflections from the colors printed on the print sheet are measured by a calorimeter such as a spectrometer, for example, thereby producing the calorimetric data Xi, Yi, Zi.
According to the process which uses the Neugebauer's equation, the calorimetric data Xi, Yi, Zi are multiplied by a coefficient of halftone-dot area percentage data hi, as shown below, thus producing the tristimulus value data X, Y, Z in the step F4. ##EQU1## where c, m, y, k represent halftone-dot area percentage data aj of the colors C, M, Y, K.
The tristimulus value data X, Y, Z thus generated are supplied to the digital color printer DP. The digital color printer DP converts the tristimulus value data X, Y, Z into data of the three primaries with respect to the LED or the like, i.e., image data depending on the devices, which may also be called inherent color space data, based on a look-up table (LUT), and thereafter generates the color proof CPa, which is a hard copy with an image formed on a sheet, based on the image data.
In the case where the tristimulus value data X, Y, Z for the digital color printer DP are generated according to the Neugebauer's equation, the colors of a printed color document to be produced can accurately be reproduced in the image on the hard copy because the colorimetric data measured by a colorimeter as representing the colors of an image to be formed on the printed color document by the color printing machine are employed. However, interference fringes such as moire, a rosette image, or the like (hereinafter referred to as a "false pattern") which appears on a printed color document, or stated otherwise an interference irregularity caused by a periodic structure of halftone dots peculiar to printed documents, cannot be reproduced in the image on the hard copy.
If such a false pattern actually appears on a printed color document, then it should also accurately be reproduced on a color proof CPa. Generally speaking, the conventional color proof CPa which fails to reproduce a false pattern thereon cannot be said as an accurate proof for a printed color document.
It is believed that no false pattern can be reproduced on a hard copy produced by the digital color printer DP because the Neugebauer's equation is a formula based on a kind of theory of probability and is unable to reproduce a microscopic image structure (halftone structure) of a false pattern.
For reproducing an image structure, it is necessary for an image output device which outputs a hard copy to have a mechanism for producing the same image structure (threshold matrix, bit map data, or the like) as that of a printed document which is to be approximated. It is difficult and substantially expensive for such a mechanism to cope with all of various printing conditions.
In order to generate an accurate proof image for printed color documents, it is not enough to reproduce the above image structure. For example, there are available various sheets and inks for printing depending on how printed documents are used and also on the user. Some color printers can use materials which are identical or very similar to those of printed documents, but find highly limited applications. It is also necessary to generate proof images in view of environments in which to compare printed documents and proof images. For example, the colors of printed documents or proof images may look different under different light sources used for observation.
The digital color printer DP for generating color proof images may not be able to reproduce desired colors depending on initial settings or aging. To correct the relationship between image data in the form of halftone dot area percentage data and colors that are outputted, it has been proposed to generate monochromatic halftone charts of the primary colors C, M, Y, measure the densities of the monochromatic halftone charts, and feed back the measured densities to generate a color conversion table for outputting desired colors, as disclosed in Japanese laid-open patent publication No. 56-141673. According to the proposed process, since no gray balance adjustments are made, the accuracy of the color of gray may not necessarily be ensured though monochromatic gradations can be reproduced with high precision. Specifically, the color of gray is produced by a superposition of plural individual colors, and even if the individual colors are established accurately and the color of gray can accurately be obtained from a theoretical standpoint, the color of gray may not accurately be obtained actually due to trapping and a dot gain upon printing because the proportions of the individual colors tend to varies thereby.
According to another conventional process, as revealed in Japanese laid-open patent publication No. 6-237373, any difference between the colors of gray between two output devices is corrected using a corrective matrix which is established to equalize equivalent neutral densities (END). This process allows the gray balance of one output device to be adjusted with respect to another output device, used as a reference, for which the color of gray has been established accurately.
With the above process, however, it is difficult to adjust the gray balance with high precision because the difference between the colors of gray is approximately corrected linearly using the corrective matrix regardless of the fact that image data are converted into output control data by a nonlinear color conversion relationship.
In either of the above conventional processes, the gray balance of an output device used as a reference is established by outputting charts based on device data of the output device and determining a conversion relationship capable of making the charts gray as a gray balance on a trial-and-error basis. Therefore, it has been quite time-consuming to establish the gray balance.
Output devices for outputting images, such as digital color printers, may undergo trouble known as shading, resulting in color irregularities on a print sheet, when an image is recorded on the entire area of the print sheet based on uniform image data. In an output device which records an image by scanning a print sheet on a drum with a laser beam, such shading may be caused by density differences due to different spot shapes of the laser beam depending on the position on the drum. In an output device which records an image on a print sheet on a drum through thermal transfer recording, such shading may be caused by density differences due to temperature irregularities in the axial direction of the drum.
The shading may be corrected by forming an image based on uniform image data over the entire area of a print medium and generating a color conversion table such that the density of the image is constant irrespective of the position on the print medium, according to the C, M, Y. The conventional process disclosed in Japanese laid-open patent publication No. 56-141673. However, such shading correction cannot be made with respect to the color of gray which is a tertiary color produced from the colors C, M, Y. The conventional shown in Japanese laid-open patent publication No. 6-237373 serves to calibrate the color of gray, but not to correct shading. Any of the above conventional processes requires a very large number of measuring points for highly accurate shading correction, and needs a considerable processing time for shading correction.